The present invention relates to novel non-metallic bearing compositions. More particularly, the invention relates to polyetherimide-based non-metallic bearing compositions.
Virtually all machinery which contains moving parts utilizes bearings to reduce wear between stationary and moving components. The most common types of bearings are roller bearings, which employ balls or needles retained by a cage with an inner and outer race. These bearings are relatively complex, require considerable lubrication and are noisy. They are also relatively bulky and heavy, which precludes their use in certain applications.
Alternatives to roller bearings are journal bearings or bushings and thrust washers. When these types of bearings are made of metal, lubrication is required, and in the event of a loss of lubrication, failure is usually catastrophic. Also, start-up wear and noise levels can be high. Moreover, the relatively high weights of these bearings can be disadvantageous, particularly in transportation applications.
One approach to overcoming many of the foregoing problems has been to employ self-lubricating bearings. Presently, self-lubricating bearings are of two general types: plastics modified with various lubricants and fillers, and soft, porous metals (such as bronze, lead, and aluminum) impregnated with oil. Oil impregnated metallic bearings are effective, but they are also expensive and are difficult to form into the desired shape.
Self-lubricating plastic bearing compositions are finding increasing applications. Depending upon the performance requirements of the bearing, a wide variety of plastic materials and compositions can be employed for bearing fabrication. For example, in light applications, such as small appliances and toys, unmodified conventional plastic materials, such as polycarbonates, polyethylene, polyacetal and nylon have been employed. As performance requirements become more demanding, non-metallic bearing compositions have become more exotic. Presently, a wide variety of resin materials are modified with various fillers and reinforcements to provide greater wear resistance and load-bearing properties and to reduce friction. Nevertheless, the major applications for nonmetallic bearings have remained relatively low in performance requirements. Excellent bearing compositions have, however, been prepared from thermosetting and certain thermoplastic polyimide resins reinforced with glass fibers and containing certain lubricants. Typical compositions have load-bearing capacities approaching those of oil-impregnated metallic bearings. These compositions are available from Dupont, Wilmington, Del., U.S.A., under the trademark Vespel.RTM.. These materials are relatively expensive and are not amenable to injection molding. Therefore, they must be machined to the desired shape, which contributes substantially to the cost of the finished product, and which also limits the possible physical configurations of the finished product.
It has recently been discovered that the use of fluoropolymers as modifiers in polyetherimide compositions results in high performance materials which can be easily fabricated using relatively simple injection molding techniques. U.S. patent application Ser. No. 626,728, filed July 2, 1984, now U.S. Pat. No. 4,532,054, describes such materials, in which the amount of fluoropolymer is most preferably about 15% by weight. The use of fluoropolymers, however, does inevitably cause some degradation of mechanical properties as a trade-off for greater wear resistance and lubrication. Hence, such compositions have heretofore also included reinforcements, such as glass or carbon fiber, as well as additional lubricants. While reinforcements improve some of the mechanical properties, their use is disadvantageous in certain applications, for example self-contact, where moving parts of a bearing having similar or identical composition are in contact with each other.
There is a continuing need, therefore, for bearing compositions which can be used in self-contact and other applications where reinforcements and fillers are prohibited. Such compositions should also possess mechanical properties approaching those of unmodified polyetherimides.